Verification of transmit power levels in a signal point limited transmission system

ABSTRACT

A pulse code modulation modem system utilizes the same total average transmit power formula for designing signal point constellations and for verifying that the transmit power of the signal point constellations are within a designated maximum power limit. The transmit power of the signal point constellations designed by the client modem is also calculated by the server modem to verify that the transmit power limit imposed upon the server modem is not exceeded. In addition, the modem system is capable of designating a transmit power limit associated with one or more training signal points or a training sequence for use during a training mode.

REFERENCE TO RELATED DOCUMENTS

This application is a continuation application of U.S. Pat. ApplicationSer. No. 09/075,719, filed May 11, 1998, now U.S. Pat. No. 6,163,570 andentitled “METHODS AND APPARATUS FOR VERYING TRANSMIT POWER LEVELS IN ASIGNAL POINT LIMITED TRANSMISSION SYSTEM”.

FIELD OF THE INVENTION

The present invention relates generally to the regulation of transmitpower levels in a signal point limited data transmission system. Inparticular, the present invention relates to a data communication systemthat verifies whether the total average transmit power of a set ofsignal point constellations is within a designated transmit power limit.

BACKGROUND OF THE INVENTION

56 kbps modem systems are quickly becoming the preferred choice amongconsumers (for client-end modems) and internet service providers (forserver-end modems). 56 kbps modem systems employ pulse code modulation(PCM) technology to facilitate higher downstream transmission data ratesto an end user. For example, FIG. 1 depicts a conceptual diagram of atypical 56 kbps communication path using current PCM modem technology. Acentral site, such as an internet service provider (ISP) 100, isdigitally connected to a telephone network 130 through a transmitter 110and a receiver 120 resident at an ISP modem 105. The network 130 isconnected to a local loop 150 through a central office line card 140.The line card typically has a PCM codec implemented therein. The localloop 150 is connected to the user's personal computer (PC) 170 at theuser's site through the user's modem 160. As can be appreciated by thoseskilled in the art, the connection between the ISP modem transmitter 110to the telephone network 130 is a digital connection that supports atypical data rate of about 64 kbps. Since the parameters of thetelephone network 130 and line card 140 are dictated and set by theoperating specifications of the network (and particularly the use of theμ-law or A-law signal point constellations), the central sitetransmitter 110 is configured to transmit the digital data in aparticular way to fully exploit its digital connection to the network.

Transmission power limitations for telecommunication systems (includingmodem systems) may be mandated by regulatory bodies such as the FederalCommunications Commission (FCC). For example, current FCC regulations onmodem transmissions over the public telephone network in the UnitedStates require that average power levels do not exceed −12 dBm0.Accordingly, the particular codewords associated with each transmissionsession, and the manner in which such codewords are transmitted, may beselected to ensure that a specific transmit power level is not exceeded.On the other hand, unnecessarily low power levels may cause a low systemsignal to noise ratio (SNR), which can result in an increasedprobability of errors and otherwise poor system performance.

In general, modem or other data communication systems that are notlimited to the transmission of specific signal points may address thetransmit power limitations in a relatively straightforward manner. Forexample, a modem system that is not limited to a particular set oftransmit signal points may simply scale its output to comply with anyregulatory restrictions. In contrast, in signal point limited systems(that may utilize signal point constellations designated by a receiver),no such scaling is possible and the constellation itself dictates thetotal average transmit power. Accordingly, the constellations designedby the receiver determine whether the transmitter complies with thetransmit power regulations.

As mentioned above, regulatory bodies may place limits on the totalaverage power utilized for a given data communication session. Becausethe transmit power limitations may vary from country to country, thedigital modem in a 56 kbps system may initially provide the maximumtransmit power limit to the analog modem such that the analog modem candesign an appropriate signal point constellation set. Accordingly, afterthe appropriate signal point constellations are selected, the totalaverage transmit power may be computed by the analog modem to ensurethat the transmit power of the constellation set does not exceed thepower limit. However, without an independent verification of thetransmit power associated with the signal point constellations, thedigital modem may utilize a signal point constellation set that, due tocomputational errors on the part of the analog modem, exceeds themaximum transmit power limit.

As mentioned above, conventional 56 kbps modem systems performconstellation design and power calculation at the analog modem (i.e.,the client-end modem) after obtaining a maximum transmit power limitfrom the digital modem (i.e., the server-end modem). Unfortunately, themanner in which the analog and digital modems calculate the totalaverage transmit power may vary from one device to the next. In otherwords, the same transmit power formula may not be rigidly followed byall modem devices. Consequently, the analog and digital modems maygenerate inconsistent transmit power calculations for the same signalpoint constellations.

Even if the analog and digital modems are in agreement with respect tothe transmit power formula, practical operating limitations (such asprocessor bit resolution or the use of finite precision arithmetic) mayintroduce round off errors to the power verification procedure. Thus,like the above situation where two different transmit power formulas areemployed, the analog and digital modems may obtain different transmitpower results for the same signal point constellation set. Thecalculation of different results utilizing the same transmit powerformula may adversely affect any verification routine later performed bythe digital modem. For example, the digital modem may reject signalpoint constellations for exceeding the transmit power limit even thoughthe analog modem designed the constellations to be within the transmitpower limit and even though the ananlog modem may have already performedan initial verification.

Present 56 kbps modem systems may not consider transmission power levelsduring training procedures. For example, training sequences may bedesignated in advance without regard to any transmit power limitationsimposed on the digital modem. Furthermore, present systems may noteffectively select the training signal points in accordance with currentoperating conditions such as the presence of robbed bit signaling ordigital pads. Such digital impairments may have a negative affect on thequality of the training procedure, especially if the training signalpoints are influenced by the digital impairments.

SUMMARY OF THE INVENTION

Accordingly, it is an advantage of the present invention that animproved data communication system for performing signal point limitedtransmissions is provided.

Another advantage of the present invention is that it provides atransmit power verification procedure that enables one modem device toverify the transmit power computation of another modem device.

Another advantage is that the total average transmit power of a signalpoint constellation set is calculated by both modem devices using thesame power formula.

A further advantage of the present invention is that it provides atransmit power verification scheme that accurately verifies the transmitpower of a signal point constellation set regardless of thecomputational resolution of the components used in the two modemdevices.

Another advantage is that the data communication system is capable ofdesignating a transmit power level for a signal point training sequenceused during a training mode.

The above and other advantages may be carried out in one form by a datacommunication system for performing signal point limited transmissions.The data communication system includes a first device and a seconddevice configured to communicate with one another over a communicationchannel, means for selecting a signal point constellation having a firstcomputed transmit power less than or equal to a transmit power limit,and means for verifying that the first computed transmit power of thesignal point constellation is less than or equal to the transmit powerlimit, where the first device includes the means for verifying. Thefirst computed transmit power is calculated by the second device inaccordance with a predetermined power calculation formula.

In accordance with another aspect of the present invention, the datacommunication system includes means for sending a transmit power limitfrom the first device to the second device, and means for selecting atraining signal point amplitude having an average transmit power lessthan or equal to a transmit power limit, where the second deviceincludes the means for selecting.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention may be derived byreferring to the detailed description and claims when considered inconnection with the Figures, where like reference numbers refer tosimilar elements throughout the Figures, and:

FIG. 1 is a block diagram of an exemplary pulse code modulation (PCM)modem system that may incorporate the present invention;

FIG. 2 is a block diagram of an exemplary PCM modem system configured toperform the transmit power verification procedures of the presentinvention;

FIG. 3 is a flow diagram of an exemplary training point selectionprocess that may be performed by the modem system shown in FIG. 2; and

FIG. 4 is a flow diagram of an exemplary transmit power verificationprocess that may be performed by the modem system shown in FIG. 2.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The present invention may be described herein in terms of functionalblock components and various processing steps. It should be appreciatedthat such functional blocks may be realized by any number of hardwarecomponents configured to perform the specified functions. For example,the present invention may employ various integrated circuit components,e.g., memory elements, digital signal processing elements, look-uptables, and the like, which may carry out a variety of functions underthe control of one or more microprocessors or other control devices. Inaddition, those skilled in the art will appreciate that the presentinvention may be practiced in any number of data communication contextsand that the modem system described herein is merely one exemplaryapplication for the invention. Further, it should be noted that thepresent invention may employ any number of conventional techniques fordata transmission, control signaling, signal processing andconditioning, and the like. Such general techniques are known to thoseskilled in the art and will not be described in detail herein.

An exemplary PCM modem system 102 that may incorporate the principles ofthe present invention is generally shown in FIG. 1, and FIG. 2 is a moredetailed block diagram depiction of a PCM modem system 200 configured inaccordance with the present invention. It should be appreciated that theparticular implementation shown in FIG. 2 and described herein is merelyexemplary and is not intended to limit the scope of the presentinvention in any way. Indeed, for the sake of brevity, conventionaltiming recovery, automatic gain control (AGC), synchronization,training, and other functional aspects of modem system 200 are notdescribed in detail herein. Furthermore, the connecting lines shown inFIG. 2 are intended to represent exemplary functional relationshipsand/or physical couplings between the various elements. Those skilled inthe art will recognize that many alternative or additional functionalrelationships or physical connections may be present in a practicalmodem system.

Modem system 200 is preferably configured as a signal point limitedtransmission system. In other words, modem system 200 may be limited tothe transmission of specific, predetermined signal points contained in asignal point constellation. For example, the μ-law and A-law signalpoint constellations are commonly used in the context of PCM modemsystems. Indeed, according to current standard operating protocols, all56 kbps modem systems operate as signal point limited systems.

Generally, modem system 200 includes a first modem, e.g., modem 202, anda second modem, e.g., modem 204. Modems 202, 204 are generallyconfigured in accordance with known principles to communicate over thepublic switched telephone network (PSTN) 205 via at least onecommunication channel, e.g., channels 206, 208. In the context ofcurrent 56 kbps modems, channel 206 may be considered to be a digitalchannel and channel 208 may be considered to be an analog or partiallyanalog channel.

Although not shown in FIG. 2, each of modems 202, 204 may include asuitable processor configured to carry out various tasks associated withthe operation of modem system 200. Indeed, modem system 200 mayincorporate any number of processors or control elements as necessary tosupport its operation. Such processors or control elements may suitablyinteract with other functional components of modems 202, 204 to therebyaccess and manipulate data or monitor and regulate the operation ofmodem system 200.

First modem 202 preferably includes an encoder 210 configured to encodedigital data in accordance with the particular encoding protocolemployed by modem system 200. For example, multiple modulus conversionmapping and μ-law or A-law signal point assignments may be used inconventional modem systems in accordance with various known andproprietary techniques. The output signal generated by encoder 210 mayinclude information for transmission during a data mode, synchronizationor training signals for transmission during an initialization mode, orcontrol or other signaling data employed by modem system 200. A signalpoint constellation database 212 (containing the particular signalpoints utilized for the current communication session) may be associatedwith encoder 210, as depicted in FIG. 2. It should be appreciated thatdatabase 212 need not be an integral part of encoder 210 and that modem202 may implement database 212 in a different manner than that shown.

Modem 202 includes a transmitter 214, which is configured to transmitencoded symbols in accordance with general PCM techniques. Such symbolsmay include data, training signals, synchronization signals, controlsignals, and the like. Modem 202 also includes a receiver 215, which ispreferably configured in accordance with conventional modemtechnologies. Receiver 215 is configured to receive data from modem 204;such data may include encoded information bits, control signals,functional parameters or identifiers, and any other data employed byconventional modem systems. For example, and as described in more detailbelow, modem 204 may be configured to send information indicative ofoptimized signal point constellations to modem 202 for use duringtransmission of subsequent signal segments. Of course, modem 202 mayemploy any suitable alternative device or technique for receiving theoptimized signal point constellations from modem 204. A decoder (notshown) resident at modem 202 may be used to decode any signalstransmitted from modem 204 to modem 202, including the signal thatconveys the signal point constellations.

In accordance with one aspect of the present invention, modem 202 isalso configured to send modem 204, via transmitter 214, a transmit powerlimit that may govern one or more training signal points or one or moresignal point constellations. As mentioned above, the transmit powerlimit may be associated with a regulatory limit imposed upon modem 202,e.g., the −12 dBm0 FCC limit. In addition, modem 202 may suitablygenerate the transmit power limit to compensate for a computationaltolerance of modem 202 (and/or modem 204), operational characteristicsof the communication channels, or the like, to ensure that the powerverification techniques of the present invention are carried out in aneffective and robust manner.

The transmit power limit may be calculated by a transmit power limitgenerator 216, which preferably uses a designated transmit power formula218 known by both modems 202, 204. The use of the designated powerformula 218 may be desirable to ensure that modems 202, 204 performcompatible power calculations that can be compared in a meaningfulmanner. In accordance with an alternate embodiment of the presentinvention, modem 202 is suitably configured to select power formula 218from a plurality of formulas. In this manner, modem system 200 iscapable of selecting a power formula expression to contemplate the useof line coding, spectral shaping, or other current operatingcharacteristics or functional features of the telephone network or modemsystem 200. In this alternate embodiment, modem 202 may suitablycommunicate the selected transmit power formula to modem 204 during aninitialization or startup procedure. It should be appreciated that thepower formula selection may instead be carried out by modem 204. Forexample, the transmit power formula, or a suitable transmit powerformula identifier, may be communicated between modems 202 and 204.

The transmit power limit may be utilized by modem system 200 as a designparameter during the selection and verification of signal pointconstellations and/or training signal points having particularamplitudes. Consequently, the transmit power formula 218 may also beutilized by a transmit power verification element 220. Transmit powerverification element 220 is configured to verify that the total averagetransmit power of the signal point constellations designed by modem 204is less than or equal to the relevant transmit power limit. Thisverification feature enables modem 202 to verify the transmit powerassociated with the constellations rather than merely assuming that theconstellations are within the transmit power limit. Furthermore, becauseboth modems 202, 204 utilize the same formula for calculating thetransmit power, the likelihood of an erroneous verification is reduced.It should be noted that, depending upon the specific application, systemtolerances, or the like, transmit power verification element 220 may beconfigured to accept or reject signal point constellations on the basisof any suitable threshold or comparison relative to the transmit powerlimit, and that the conditions for verification described herein aremerely exemplary.

Transmit power verification element 220 may include or be operativelyassociated with a transmit power calculation element 222 and acomparator element 224. Power calculation element 222 is preferablyconfigured to calculate, in accordance with the predetermined powercalculation formula 218, a computed transmit power of the signal pointconstellations designed by modem 204. As mentioned above, transmit powerconsiderations can play an important role during adaptive signal pointconstellation design and various optimization techniques. Accordingly,modem systems must be designed to operate in a robust and reliablemanner while maintaining the total average transmit power within thegiven limit. For this reason, if the transmit power of a constellationexceeds the power limit, then the modem system 200 may redesign theconstellation such that its total average power is within an acceptablerange.

Although the present invention can be used in the context of anytransmit power formulation, the preferred 56 kbps modem embodimentemploys a power formula 218 that contemplates the unequal probability ofoccurrences associated with the signal points within the signal pointconstellations. Generally, the power calculation initially determinesthe probability of transmission of each individual signal point within aparticular constellation. Then, the probabilities of occurrence are usedto generate the average total power for each constellation (current 56kbps modem systems use six signal point constellations associated withdifferent data frame segments). Finally, the power for all of theconstellations is determined and averaged to produce the total averagepower for the given data communication session. An exemplary exact powercalculation scheme suitable for use with modem system 200 is describedin detail in U.S. patent application Ser. No. 09/013,671, filed Jan. 26,1998; the entire content of this patent application is incorporatedherein by reference.

Comparator element 224 is suitably configured to determine whether thetotal average transmit power calculated by power calculation element 222is less than or equal to the transmit power limit. Accordingly, transmitpower verification element 220 may receive the transmit power limitfrom, or otherwise operate in conjunction with, transmit power limitgenerator 216. Power verification element 220 is preferably configuredto accept or reject the signal point constellations provided by modem204 in response to the determination made by comparator 224 (describedin more detail below).

As described above, the transmit power limit may be utilized by modemsystem 200 to guide the selection of training signal point magnitudes bymodem 204. Accordingly, modem 202 may include a control and processingelement 226 associated with the generation of training sequences thatcontain the training point amplitudes selected by modem 204. Control andprocessing element 226 is preferably configured to control the operationof a training sequence generator 228. In the preferred embodiment, onetraining signal point amplitude is utilized in a training sequence, andtraining sequence generator 228 may format the training sequence suchthat it includes a plurality of signal points corresponding to positiveand negative values of the particular training point amplitude. Thus,training sequence generator may include a suitable polarity assigner(not shown) that assigns a positive or a negative polarity to thetransmitted training symbols.

With continued reference to FIG. 2, signals transmitted to modem 204over channel 206 are received by a receiver 230. It should be noted thatreceiver 230 may include any number of additional components (that maybe known in the art) for decoding, equalization, conditioning, or otherprocessing of the received signal. Modem 204 processes received datasignals to obtain the original digital data encoded by modem 202; modem204 includes a decoder 232 to suitably decode the received symbols inaccordance with the same encoding scheme employed by encoder 210. Aswith encoder 210, decoder 232 may have a signal point constellationdatabase 234 associated therewith. Database 234 is preferably utilizedto store the same signal point constellations that are stored at andutilized by modem 202 for the current communication session.

Modem 204 may include a receiver training control block 236 thatinitiates and regulates training or resynchronization processes withinmodem 204. As shown in FIG. 2, receiver training control block 236 maybe associated with receiver 230 to enable the adaptive equalization ofreceiver 230 in response to the training sequence sent by modem 202. Asdescribed above, prior to the training procedure, modem 204 may receivea predetermined transmit power limit from modem 202. Modem 204 may thenaccess a signal point selector 238 to determine which training signalpoint amplitude would be suitable for the given power limit. For complextraining sequences that include more than one signal point, modem 204may utilize a transmit power calculation element 242 (that uses adesignated transmit power formula 240) to suitably calculate thetransmit power associated with the training signal points or theproposed training sequence itself.

Signal point selector 238 is also configured to select at least onesignal point constellation (and preferably a set of constellations) suchthat the signal point constellation set has a computed transmit powerless than or equal to the transmit power limit identified by modem 202.Power calculation element 242 computes the total average power of thesignal point constellations in accordance with the designated powerformula 240 and in a similar manner as transmit power calculationelement 222 (resident at modem 202). In practice, modem 204 is the firstdevice to compute the total average transmit power of the signal pointconstellation. Modem 202 subsequently verifies that the computedtransmit power is indeed within the specified transmit power limit. Ifthe calculation performed by modem 202 determines that a constellationset designed by modem 204 exceeds the transmit power limit, then thatconstellation set is either discarded or modified by signal pointselector 238 until an acceptable constellation set is obtained.

A transmitter 244 is preferably utilized to send information indicativeof the designated training signal point amplitude or amplitudes to modem202. In the preferred embodiment, the information transmitted bytransmitter 244 is encoded prior to transmission over PSTN 205. Uponreceipt of this information, modem 202 performs decoding and processingto obtain the training point amplitudes for subsequent use by trainingsequence generator 228. Then, during a training mode, a suitabletraining sequence is sent from modem 202 to modem 204. Thereafter, thesignal point constellations (within the designated transmit power limit)designed by modem 204 are transmitted, via transmitter 244, to modem202. Modem 202 suitably extracts the signal point constellations for thetransmit power verification procedure. If modem 202 verifies that thetransmit power associated with the signal point constellations is withinthe designated transmit power limit, then a data communication sessioncan be initiated.

Referring now to FIG. 3, an exemplary training point selection process300 is illustrated as a flow diagram. Although not depicted as such,specific portions of process 300 may be performed by modem 202 or modem204. Process 300 is preferably performed by modems 202, 204 during aninitialization or startup procedure after communication channels 206,208 have been established. Process 300 may begin with a task 302, duringwhich modems 202, 204 enter a training mode. Task 302 may be prompted bya certain control signal sequence transmitted by modem 202.Alternatively, task 302 may follow a predetermined sequence ofinitialization or startup routines, where such sequencing issynchronized at both modems 202, 204.

After a training mode is established between modems 202, 204, a task 304may be performed to cause modem 202 to generate a transmit power limitfor application during the selection of one or more training signalpoints. As described above in connection with FIG. 2, the transmit powerlimit may be suitably generated in response to a computational toleranceof modem 202 (and/or modem 204), a regulatory limit imposed upontransmissions by modem 202, or operational characteristics ofcommunication channels 206 or 208. Regulatory limits that rarely change,such as the −12 dBm0 FCC power limit described above, may be stored in asuitable memory element for access by transmit power limit generator216. Alternatively, modem 202 may generate new transmit power limits foreach communication session to compensate for other functionalparameters. For example, modem 202 may generate a lower transmit powerlimit to compensate for the nonlinearity of network codecs present inchannels 206, 208. In addition, modem 202 may lower the transmit powerlimit to ensure that its computational precision does not cause anerroneous acceptance or rejection of training points or a signal pointconstellation set designed by modem 204.

Following task 304, a task 306 is preferably performed to cause modem202 to send the current transmit power limit to modem 204. In addition,modem 204 receives the current transmit power limit during task 306.Modem 204 uses the received transmit power limit to select at least onesuitable training signal point (task 308). As described above inconnection with FIG. 2, signal point selector 238 is preferablyconfigured to perform task 308. In the preferred embodiment, onetraining point amplitude is selected during task 308, i.e., theresultant training signal is a sequence of positive and negative symbolscontaining the specified signal point amplitude.

Modem 204 selects a training signal point having an average transmitpower less than or equal to the transmit power limit. In addition, modem204 may consider any number of factors during the selection of thetraining signal point. Such flexibility may be desirable to enable modemsystem 200 to contemplate practical limitations associated with thecurrent communication session. For example, the selection of thetraining signal amplitude may be biased toward those signal pointshaving relatively higher average transmit powers. The use of higherpower signal points is desirable to increase the signal to noise ratioassociated with the subsequent transmission of the training sequence.This additional consideration may also be associated with the design ofthe signal point constellations used during the data mode.

The selection of the training point may also be responsive to thepresence of a digital impairment, e.g., robbed bit signaling (RBS),within communication channel 206. In this context, the training pointmay be selected to either reduce the effect of RBS within the receivedtraining sequence or to increase the detection of RBS within thereceived training sequence. Those skilled in the art will appreciatethat, because digital impairments such as RBS may affect a specificframe segment or a particular bit within a transmitted codeword, anynumber of processing techniques may be employed to address the presenceof RBS. For example, the training signal point amplitude (or acorresponding codeword) may be represented by a number of digital bits,and signal point selector 238 may be suitably configured to select thetraining signal point amplitude in accordance with a predetermined bitassignment for at least one of the digital bits that represent thesignal point amplitude. For example, because RBS typically replaces theleast significant bit of a codeword with a “1”, task 308 may be designedto select a training signal point amplitude that is represented by acodeword having a “1” as its least significant bit (such a selectionwould reduce the effect of RBS in the training sequence). On the otherhand, if the objective is to easily detect the presence of RBS in thetraining sequence, then task 308 may instead select a training signalpoint amplitude that is represented by a codeword having a “0” as itsleast significant bit.

After the selection of the training point by modem 204, a task 310 isperformed. During task 310, the specific training signal point amplitude(or data indicative thereof) is sent from modem 204 to modem 202 in anappropriate manner. In addition, modem 202 suitably receives thetraining signal point amplitude during task 310. As described above,training sequence control and processing element 226 (see FIG. 2) mayreceive the training signal point amplitude and thereafter control theoperation of training sequence generator 228. Accordingly, a task 312 ispreferably performed to generate a training sequence that includes aplurality of signal points corresponding to the specified amplitude. Asdescribed above, task 312 may suitably assign positive and negativepolarities to the plurality of signal points to form the trainingsequence. The particular polarity assignment may vary for differentapplications or to accomplish different training objectives.

A task 314 causes modem 202 to transmit the training sequence overchannel 206. Modem 204 suitably receives the training sequence duringtask 314 and thereafter trains receiver 230 (and any other applicablecomponents) in response to the training sequence. Following task 314,training point selection process ends. In due course, modem system 200may enter an adaptive constellation design mode followed by a datatransmission mode.

The adaptive constellation design mode may be performed in conjunctionwith a transmit power verification process 400, which is illustrated asa flow diagram in FIG. 4. Portions of process 400 may be performed bymodem 202 while other portions of process 400 may be performed by modem204. Process 400 may begin with a task 402, which causes modem system200 to select a transmit power calculation formula from a plurality offormulas. Task 402 may not be necessary in systems that utilize only onepredetermined power calculation formula. In other systems, task 402 maybe desirable to enable the selection of a transmit power formula that isparticularly suitable for communication channels 206, 208 or for thecurrent operating characteristics of modem system 200. If necessary,task 402 may be performed at either of modems 202, 204, and the selectedpower formula may be communicated between modems 202, 204 via a suitablytransmitted control sequence.

Next, a task 404 may be performed to generate, in accordance with theselected power calculation formula, a transmit power limit. As describedabove, task 404 is preferably performed by modem 202 and, in particular,transmit power limit generator 216 (see FIG. 2). In the preferredembodiment, task 404 generates a limit for the total average transmitpower for use by modem 204 during the adaptive design of a set of signalpoint constellations. As described above in connection with trainingpoint selection process 300 (see FIG. 3), the transmit power limit maybe suitably generated in response to a computational tolerance of modem202 (and/or modem 204), a regulatory limit imposed upon transmissions bymodem 202, or operational characteristics of communication channels 206or 208.

A task 406 causes modem 202 to send the transmit power limit (generatedduring task 404) to modem 204; modem 204 receives the transmit powerlimit during task 406. After modem 204 receives the transmit powerlimit, a task 408 is preferably performed to select a signal pointconstellation (or a set of constellations) such that the constellationset has a first computed transmit power less than or equal to the totalaverage transmit power limit (as described above, the constellation setmay be alternatively designed to satisfy any condition relative to thepower limit). The first computed transmit power is calculated inaccordance with the designated power calculation formula. Thus, bothmodems 202, 204 utilize a consistent power formula during theirrespective calculations.

In accordance with the preferred embodiment, the first computed transmitpower is associated with an upper bound calculation with respect to thepredetermined power calculation formula. In other words, the finitearithmetic precision of modem 204 is taken into consideration such thatthe first computed transmit power will not be less than a computed valueusing infinite precision arithmetic. It should be appreciated that modem204 need not always employ an upper bound calculation. For example, ifsuitable approximation power formulas are used that are known to providea result within a certain error margin, then that error margin can beadded to the result of the power calculation before it is compared tothe transmit power limit.

In a practical modem system, the selection of the specific constellationsignal points may be governed by a number of criteria other thantransmit power, e.g., network and channel characteristics. Consequently,any number of conventional techniques may be employed during task 408; adetailed explanation of such techniques, including adaptiveconstellation design, is beyond the scope of this description.

Following task 408, modem 204 has preferably designed a first computedset of signal point constellations that, according to the calculationsperformed by modem 204, have a total average transmit power less than orequal to the transmit power limit. Next, a task 410 causes modem 204 totransmit the constellations (or data indicative of the constellations)to modem 202, which suitably receives the constellations. A task 412 maybe performed to prompt modem 202 to verify that the total averagetransmit power of the constellations is less than or equal to thedesignated transmit power limit. Modem system 200 may utilizeconventional control or signaling techniques to perform task 412.

During a task 414, modem 202 verifies the total transmit power bycalculating, in accordance with the predetermined power calculationformula, a second computed transmit power of the first computed signalpoint constellations . In the preferred embodiment, the second computedtransmit power is associated with a lower bound calculation with respectto the predetermined power formula. In other words, the finitearithmetic precision of modem 202 is taken into consideration such thatthe first computed transmit power will not be greater than a computedvalue using infinite precision arithmetic. The lower bound calculationis preferably utilized to ensure that modem 202 does not rejectconstellations that were correctly designed by modem 204. It should beappreciated that modem 202 need not always employ a lower boundcalculation. For example, if suitable approximation power formulas areused that are known to provide a result within a certain error margin,then the transmit power limit can be selected at an appropriately lowerlevel to include the certain error margin.

Following task 414, a query task 416 may be performed by comparator 224(see FIG. 2) to test whether the second computed transmit power is lessthan or equal to the designated total transmit power limit. As mentionedpreviously, the specific condition analyzed during query task 416 mayvary according to the particular system. If query task 416 determinesthat the second calculated transmit power exceeds the predeterminedpower limit, then a task 418 is preferably performed to reject thecurrent set of signal point constellations. Following task 418, transmitpower verification process 400 ends. Task 418 may prompt modem 204 toperform a redesign of the constellations to lower the transmit power,i.e., process 400 may be reentered at task 408 following task 418.

If query task 416 determines that the second computed transmit powerdoes not exceed the designated total power limit, then a task 420 isperformed to accept the set of constellations designed by modem 204.Acceptance or verification of the signal point constellations may causethe constellations to be suitably stored at, e.g., constellationdatabase 212 for subsequent use during the data mode. Following task420, a task 422 may prompt modem system 200 to initiate a datacommunication session utilizing the current (and verified) signal pointconstellations. Process 400 ends after task 422. Thus, process 400ensures that modems 202, 204 both utilize the same transmit powerformula during their respective calculations, while providing a transmitpower verification procedure for modem system 200.

In summary, the present invention provides an improved datacommunication system for performing signal point limited transmissions.A modem system in accordance with the present invention performs atransmit power verification procedure that enables one modem device toverify the transmit power computation of another modem device. Thetransmit power verification scheme accurately verifies the transmitpower of a signal point constellation set regardless of thecomputational resolution of the components used in the two modemdevices. The total average transmit power of a signal pointconstellation set is calculated by both modem devices using the samepower formula. In addition, the modem system is capable of designating atransmit power level for a signal point training sequence used during atraining mode.

The present invention has been described above with reference to apreferred embodiment. However, those skilled in the art will recognizethat changes and modifications may be made to the preferred embodimentwithout departing from the scope of the present invention. For example,some aspects of the present invention may not be limited to anyparticular hardware implementation. In addition, the various processingtasks may be equivalently performed in a different order than that shownand described herein. These and other changes or modifications areintended to be included within the scope of the present invention, asexpressed in the following claims.

What is claimed is:
 1. In a first modem capable of communicating inanalog mode, a method for initiating verification of transmit powerlevels in a signal point limited transmission system, wherein said firstmodem is configured to communicate over a communication channel with asecond modem capable of communicating in a digital mode, said methodcomprising the steps of: receiving a transmit power limit; selecting atleast one signal point constellation having a first computed transmitpower less than or equal to said transmit power limit, said firstcomputed transmit power being calculated in accordance with apredetermined power calculation formula; and transmitting said at leastone signal point constellation to said second modem to initiateverification of the transmit power level of said at least one signalpoint constellation.
 2. The method according to claim 1, furthercomprising the step of initiating a data communication session utilizingsaid at least one signal point constellation.
 3. The method according toclaim 2, wherein said data communication session initiating step isperformed if said verification of said transmit power is successful. 4.The method according to claim 1, wherein said first computed transmitpower is associated with an upper bound calculation with respect to saidpredetermined power calculation formula.
 5. The method according toclaim 1, further comprising the step of selecting said predeterminedpower calculation formula from a plurality of formulas.
 6. The methodaccording to claim 5, wherein said selecting step is responsive tocurrent operating characteristics of said transmission system.
 7. Afirst modem capable of communicating in an analog mode, configured tocommunicate over a communication channel with a second modem capable ofcommunicating in a digital mode, in a signal point limited transmissionsystem, said first modem comprising: a receiver configured to receive atransmit power limit; a processor configured to select at least onesignal point constellation having a first computed transmit power lessthan or equal to said transmit power limit, said first computed transmitpower being calculated in accordance with a predetermined powercalculation formula; and a transmitter configured to transmit said atleast one signal point constellation to said second modem to initiateverification of the transmit power of said at least one signal pointconstellation.
 8. The first modem according to claim 7, wherein saidprocessor is further configured to initiate a data communication sessionutilizing said at least one signal point constellation.
 9. The firstmodem according to claim 8, wherein said processor initiates said datacommunication session if said verification of said transmit power issuccessful.
 10. The first modem according to claim 8, wherein said firstcomputed transmit power is associated with an upper bound calculationwith respect to said predetermined power calculation formula.
 11. Thefirst modem according to claim 7, wherein said predetermined powercalculation formula is selected from a plurality of formulas.
 12. Thefirst modem according to claim 11, wherein said predetermined powercalculation formula is selected in accordance with current operatingcharacteristics of said transmission system.
 13. A first modem capableof communicating in an analog mode, configured to communicate over acommunication channel with a second modem capable of communicating in adigital mode, said first modem comprising: means for receiving atransmit power limit; means for selecting at least one signal pointconstellation having a first computed transmit power less than or equalto said transmit power limit, said first computed transmit power beingcalculated in accordance with a predetermined power calculation formula;and means for transmitting said at least one signal point constellationto said second modem to initate verification of the transmit power ofsaid at least one signal point constellation.
 14. The first modemaccording to claim 13, wherein said means for selecting includes meansfor initiating a data communication session utilizing said at least onesignal point constellation.
 15. The first modem according to claim 14,wherein said means for selecting includes means for initiating said datacommunication session if said verification of said transmit power issuccessful.
 16. The first modem according to claim 13, wherein saidfirst computed transmit power is associated with an upper boundcalculation with respect to said predetermined power calculationformula.
 17. The first modem according to claim 13, wherein saidpredetermined power calculation formula is selected from a plurality offormulas.
 18. The first modem according to claim 13, wherein saidpredetermined power calculation formula is selected in accordance withcurrent operating characteristics of said transmission system.
 19. Acomputer program product, said computer program product comprising: codefor enabling access to a transmit power limit of the data transmissionsystem; code for selecting, at a first modem capable of communicating inan analog mode, at least one signal point constellation such that thesignal point constellation has a first computed transmit power less thanor equal to said transmit power limit; and code for transmitting said atleast one signal point constellation to a second modem capable ofcommunicating in a digital mode to initiate verification of the transmitpower of said at least one signal point constellation.
 20. The computerprogram product of claim 19, wherein said at least one signal pointconstellation comprises PCM signal points.
 21. The computer programproduct of claim 19, wherein said first computed transmit power iscalculated in accordance with a power calculation formula.
 22. Thecomputer program product of claim 21, wherein said first computedtransmit power is associated with an upper bound calculation withrespect to said power calculation formula.
 23. The computer programproduct of claim 19, further comprising code for calculating saidtransmit power limit in accordance with a power calculation formula. 24.The computer program product of claim 23, wherein said code forcalculating is configured to calculate a total average transmit powerlevel.
 25. The computer program product of claim 19, wherein the codefor determining said transmit power limit resides at said second modem.26. The computer program product of claim 25, wherein the code fordetermining the transmit power limit further comprises code for enablingcommunication the transmit power limit between the first modem and thesecond modem.
 27. A computer program embodying instructions executableby a computer to perform method, the method steps comprising: generatinga transmit power limit for the system; determining a set of signalpoints; verifying said set set of signal points; and storing said set ofsignal points in a database if said verifying step confirms a secondtransmit power is less than or equal to the transmit power limit. 28.The computer program of claim 27, wherein said generating a transmitpower limit for the system step is executed by performing one of thesteps of: retrieving a stored transmit power limit from a memoryelement; and calculating said transmit power limit in accordance with apredetermined transmit power formula.
 29. The computer program of claim27, wherein said determining a set of signal points steps includes oneof the steps of: calculating a transmit power associated with said setof signal points in accordance with a predetermined transmit powerformula; and selecting said set of signal points such that saidcalculated transmit power is less than or equal to said transmit powerlimit.
 30. The computer program of claim 27 wherein said verifying saidset of signal points step includes the steps of: calculating a secondtransmit power associated with said set of signal points in accordancewith a predetermined transmit power formula; and determining whethersaid second transmit power is less than or equal to said transmit powerlimit.
 31. A computer data signal representing sequences of instructionswhich, when executed by a modem system, enables said modem system to:provide for the determination of a transmit power limit for said system;provide for the selection of a set of signal point constellations tosatisfy a condition relative to said transmit power limit; and providefor the verification that said set of signal point constellationssatisifies said condition relative to said transmit power limit.
 32. Thecomputer data signal of claim 31, wherein said data signal furtherenables said modem system to provide for the querying of whether saidcondition was satisfied.
 33. The computer data signal of claim 31,further enabling the modem system to: provide for the rejection of saidset of signal point constellations when the condition is not satisfied;and provide for the acceptance of said set of signal pointconstellations when the condition is satisfied.
 34. The computer datasignal of claim 31, further enabling the modem system to provide for theinitiation of a data communication session utilizing the verified set ofsignal point constellations.
 35. The computer data signal of claim 31,wherein said step of establishing a transmit power limit for said systemincludes selecting a maximum transmit power from a table stored in amemory.
 36. A machine-readable program storage medium having a programstored thereon, said program configured to enable performance of thefollowing steps: determining a transmit power limit; selecting at leastone signal point constellation; and communicating said at least onesignal point constellation to initiate verification of the transmitpower of said at least one signal point constellation.
 37. Themachine-readable program storage medium of claim 36, wherein selectionof said at least one signal point constellation is such that said atleast one signal point constellation has s first computed transmit powerless than or equal to said transmit power limit, said first computedtransmit power being calculated in accordance with a predetermined powercalculation formula.
 38. The machine-readable program storage medium ofclaim 36, wherein said program is further configured to enableperformance of the step of initiating a data communication sessionutilizing said at least one signal point constellation.
 39. Themachine-readable program storage medium of claim 38, wherein saidinitiating step is performed if said verification of said transmit poweris successful.
 40. The machine-readable program storage medium of claim36, wherein said first computed transmit power is associated with anupper bound calculation with respect to said predetermined powercalculation formula.
 41. The machine-readable program storage medium ofclaim 36, wherein said program is further configured to enableperformance of the step of selecting said predetermined powercalculation formula from a plurality of formulas.
 42. Themachine-readable program storage medium of claim 41, wherein saidselecting step is responsive to current operating characteristics ofsaid transmission system.
 43. The machine-readable program storagemedium of claim 36, wherein said program is stored in a random accessmemory (RAM).
 44. The machine-readable program storage medium of claim36, wherein said program is stored in a read-only memory (ROM).
 45. Themachine-readable program storage medium of claim 36, wherein saidprogram is stored in anelectrically-erasable-and-programmable-read-only-memory (EEPROM). 46.The machine-readable program storage medium of claim 36, wherein saidprogram is stored in a memory region provided within a digital signalprocessor.
 47. The machine-readable program storage medium of claim 36,wherein said step of determining a transmit power limit includesreceiving a five-bit binary number corresponding to said transmit powerlimit.
 48. A computer data signal for establishing a data communicationsession between a first modem capable of communicating in an analog modeand a second modem capable of communicating in a digital mode, saidcomputer data signal comprising: instructions capable of enabling saidfirst modem to receive a transmit power limit from said second modem;instructions capable of enabling said first modem to select a signalpoint constellation such that a first computed transmit power, ascalculated by said first modem, is within said transmit power limit;instructions capable of enabling said first modem to transmit saidsignal point constellation to said second modem such that said secondmodem is capable of verifying that siad first computed transmit power iswithin said transmit power limit, and capable of initiating a datacommunication session utilizing said signal point constellation uponverification that said first computed transmit power is within saidtransmit power limit, or capable of rejecting said signal pointconstellation upon verification that said first computed transmit poweris not within said transmit power limit.
 49. The computer data signal ofclaim 48, wherein said first computed transmit power is less than orequal to said transmit power limit.
 50. The computer data signal ofclaim 48, wherein said first computed transmit power is associated withan upper bound calculation with respect to a power calculation formula.51. A first modem capable of communicating in an analog mode, configuredto communicate over a communication channel with a second modem capableof communicating in a digital mode, in a signal point limitedtransmission system, said second modem comprising: means for sending atransmit power limit to said first modem; means for receiving at leastone signal point constellation from said first modem; means forcalculating a computed transmit power based on said at least one signalpoint constellation; and means for verifying that said computed transmitis less than or equal to said transmit power limit.
 52. A first modemcapable of communicating in an analog mode, configured to communicateover a communication channel with a second modem capable ofcommunicating in a digital mode, said second modem comprising: atransmitter configured to transmit a power limit; a receiver configuredto receive at least one signal point constellation from said firstmodem; and a processor configured to calculate a computed transmit powerbased on said at least one signal point constellation, and to verifythat said computed transmit is less than or equal to said transmit powerlimit.